When I write my "feelings regarding the layoff", if they wind up as an angry rant about "my stupid worthless boss" and "my goddamn no-good backstabbing coworkers", does that still work? Or do I need to be more even-handed about this, identify things I might have done wrong and reasons I might have deserved it, admit that it was mostly my fault, etc...?

I don't know the specifics of writing this sort of journal, but I do want to point out that "my stupid worthless boss" and "my goddamn no-good backstabbing coworkers" are not feelings. They are beliefs about the situation, and as such are true or false. If you anticipate believing these things in the future, you might as well go ahead and believe them now, and start getting your resume out pronto.

ETA: Something that had not occurred to me when writing that is that I'm getting laid off in about six weeks time. But I've seen it coming at least a year out. Too complicated, idiosyncratic, and irrelevant story to go into any detail about, but I've simply counted my money, counted what I want to do with my time, and am not actually inconvenienced by it.